Poly(ethylene terephthalate) (PET) films are widely used for a variety of wrapping, packaging and lamination applications. PET film sometimes is used in shrink wrap applications in which the film is applied to an object and heated so that the film shrinks around the object. In other applications such as flexible electronic circuits, heat resistant packaging, and cook-in bags, biaxially oriented and heat set PET film having good dimensional stability and shrink resistance at elevated temperatures is used. However, biaxially oriented PET films are not useful at temperatures exceeding 250° C. because their inherent melting temperature (Tm) is 250° C.
Certain applications, such as certain flexible circuit boards, require films that are heat stable (i.e., possessing good dimensional stability) at 260° C. Specifically, the films must not blister or wrinkle when immersed in a solder bath preheated to 260° C. More specifically, these films must undergo 3% or less shrinkage when immersed for 10 seconds in a solder bath preheated to 260° C. Films that meet this requirement are combined with adhesive and circuitry into a flexible laminate that can then be subjected to a wave or dip solder at 260° C. Any blistering or wrinkling of the base film of this laminate during soldering at 260° C. may affect circuit performance in the final application.
Certain high temperature flexible circuit board applications also require that the base film be attached to a copper film by the use of a thermally curable adhesive. These adhesives typically are cured under pressure at temperatures between 120–150° C., and thus the entire laminate structure must be heated to these temperatures in a hot press, autoclave or equivalent equipment. Once the adhesive is cured, the laminate is cooled to room temperature. It is important that the coefficient of thermal expansion (CTE) of the base film and adhesive be sufficiently similar to the CTE of the copper film over the temperature range from 150 to 23° C. so that residual stresses do not develop which might cause the laminate to curl. In order to prevent curl, the base film must have a CTE value of 10–85 ppm/° C., when measured between 120 and 150° C., and preferably also has a CTE value of 10–42 ppm/° C., when measured between 25 and 90° C. Note that these CTE values are specific to biaxially oriented films used to make laminates and are not required in films used for other heat resistant packaging applications, such as cook-in bags, and is not believed to have been addressed in the prior art.
Superior hydrolytic stability is another important requirement for films used in flexible circuit boards, particularly for automotive applications, and cook-in bags. Base films with superior hydrolytic stability produce circuit boards and bags that will maintain their structural integrity. In addition, it is important for the substrate and cover films used in flexible circuit boards to be insulative materials, to prevent charge from bridging across the circuitry. The insulative capability of a film used in these applications is measured by the dielectric constant. It is important for a film used in flexible circuit board applications to have as low a dielectric constant as possible.
Certain polyesters derived from 1,4-cyclohanedimethanol (CHDM) have melting points greater than 250° C. The following references discuss films made from polyesters prepared from CHDM. However, none of the patent references are believed to specifically disclose the fabrication of film that is stable at 260° C.
Defensive Publication T876,001 (1970) describes films made from poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) homopolyesters and copolyesters containing up to 20 mole percent isophthalic acid residues.
Published Patent Application WO 96/06125 (1996) describes biaxially oriented films made from PCT polymers.
Published Patent Application WO 92/14771 (1992) describes PCT polymers having an inherent viscosity (I.V.) less than 0.80 dL/g and wherein the CHDM residues consist of 75 to 100% cis isomer.
JP 1-299019 A (1989) describes biaxially oriented PCT films produced from PCT polymers containing up to 15 mole percent of residues derived from other dicarboxylic acids or other glycols and having an I. V. of at least 0.5 dL/g. JP 2-301419 (1989) describes how the biaxially oriented PCT films discussed in JP 1-299019 do not have 260° C. solder bath resistance, and describes how solder bath resistance can be obtained by irradiating the films discussed in JP 1-299019 to cross link the films. JP 2-196833 A (1990) describes an electric insulation material based on metallized biaxially oriented PCT film. CTE, use temperatures, and flexible circuit board applications are not discussed in this document.
U.S. Pat. No. 4,557,982 (1985), JP 60-069133 A (1985), JP 91-000215 B (1991), JP 60-085437 A (1985) and JP 90-063256 B (1990) describe PCT polymers containing at least 80 mole percent terephthalic acid residues and at least 90 mole percent CHDM (60 to 100% trans isomer) residues. Biaxially oriented films with high biaxial orientation are useful in making magnetic tapes.
JP 58-214208 A (1983) describes films made from PET, PCT and copolyesters useful for electrical insulation and flexible printed circuit base film. U.S. Pat. No. 5,153,302 (1992) and JP 4-214757 A (1992) describes PCT and PCT copolyesters containing at least 97 mole percent CHDM residues and at least 90 mole percent terephthalic acid residues and the use of the polymers to make capacitor films. U.S. Pat. No. 3,284,223 (1966) describes PCT polymers containing up to 25 mole percent of residues derived from other dicarboxylic acids or other glycols.